The technique of assembly by bolts has always posed and even now poses numerous problems for its users, because of the use of complex pieces or use in severe conditions. The cost of accidents which the failure of this type of assembly is liable to cause is out of all proportion to the intrinsic cost of this technique and therefore justifies the interest which is given to it.
Expertises applied following the failure of bolted assemblies, such as ruptures, leaks at clamps after assembly, show that these failures, for the most part, are due to misappreciation of the load or preload introduced into the assembly in the region of the bolts at the instant of tightening as such.
The preload by tensioning of bolts according to an array of predetermined values, a function of the structure of the assembly, constitute one of the essential parameters of the final assembly. A judicious preload permits the body of the bolt not to work in sheer in the case of forces tangential to the plane of assembly. It also permits ensuring the maintenance of fluid-tightness and conditions the effects of spontaneous slackening, under the action of dynamic sheer forces. Finally, it permits the better use of the mechanical characteristics of bolts, thus permitting a reduction of their diameter and number, in short obtaining an optimalisation of the tightening array and of the assembly. Presently, an integrated system is not available permitting the control in real time of the tension of bolts, according to a pre-determined tightening array, as a function of the structure of the assembly.